Compressibility is a term applied in helicopter aerodynamics to a phenomenon which occurs as an aerofoil approaches the speed of sound. During rotation of the rotor blades of a helicopter this occurs first at the tip area of the advancing blade due to the sum of the tangential velocity of the blade and forward velocity of the helicopter, resulting in high noise levels, a limitation of forward speed due to increased drag, and an increase in vibration and control loads.
Blade vortex interaction is a further undesirable phenomenon affecting the efficiency of helicopter rotor blades. It is caused by a vortex which leaves primarily by the tip area of a preceding blade coming into contact with a following blade. This results in what is known as impulsive loading which causes vibrations, noise, and blade fatigue.
Speed limitation together with other undesirable limitations also result from a blade stall condition which occurs in the retreating blade when relative velocities are low, due to the angle of attack of the blade having to be increased in order to maintain rotor trim.
It is known that a thinned swept tip portion on a helicopter rotor blade will reduce the adverse effects of compressibility on an advancing blade. However, in operation of a helicopter rotor, large blade torsional motions and loads are encountered on the retreating blade when an appreciable spanwise extent of the blade becomes stalled. These loads can result in a limitation of the flight envelope when they exceed the design fatigue capability of the blade or control system, even though the lifting portions of the rotor at the front and rear of the rotor disc quadrants are in fact capable of producing greater lift. The most important region of the retreating blade for producing such violent loads is the blade tip (i.e. approximately the outer 20% of the blade).
Previously, attempts to extend the performance of a helicopter rotor which is limited by a retreating blade stall condition have been concerned with maintaining an attached flow condition over the tip portion of the blade either by improved section lift capability or twist, or combinations of both. However, the thinned section requirements of the advancing blade conflict with these improvements required for the retreating blade, and increased local washout (or twist) to reduce pitch on the retreating blade produces high bending loads on the advancing blade.